i think Paul's right. the children are the major concern. i'm surprised at how much people on this site complain about the results of single parent homes and are so quick to recommend someone raise his children that way.
i suggest a good long talking to and perhaps some marital counseling. if it's at all possible to keep the marriage civl and respectable to raise the children properly, then try it.
my boyfriend was on a business trip recently and sat next to a guy that had had several affairs. so had this guy's wife. well, turns out that they both started communicated, and along with a new life in Jesus, he and his wife are happier than they've ever been. seems that the affairs had happened because neither husband nor wife knew how to talk to the other about what was bothering him/her. but the "other guy/gal" was easy to talk to about those things and seemed so caring and understanding. and a false love/infatuation grew out of being able to communicate with someone else.
perhaps your wife (assuming this isn't a hypothetical and the unfortunate situation is your reality) felt the same way. it's easy for men and women to fall into that trap.
if on the otherhand, she's just selfish and childish, it probably is best to cut your losses. regardless, still try and keep a least a talk-able relationship with her for the children.
good luck.
oh, i wanted to add: beware of taking her to the cleaners out of spite. it leads down a very bad road of eternal bitterness and antagonism (sp?). neither of which your children need to be seeing. if you have to make a break and it might get ugly, use a lawyer and don't talk to her at all. it will help keep the hostility down. sometimes it's painful and unfair being the better person, but it's worth it in the end.